custom email recipient in comments notification e-mail

WordPress Codex – comment_post

You can try this:

add_action('comment_post', 'notify_author_of_reply', 10, 2);

function notify_author_of_reply($comment_id, $approved){
  if($approved){
    $comment = get_comment($comment_id);
    if($comment->comment_parent){
      $parent_comment = get_comment($comment->comment_parent);
      wp_mail($parent_comment->comment_author_email, 'Hello', 'Dude you got a reply...');
    }
  }
}

source: https://wordpress.stackexchange.com/a/36590

alternatively, you may be interested in this plugin

Thanks for help, that plugin seems to be useful and usable.